This invention relates to a lightning arrester insulator in which a voltage non-linear resistor having a major constituent of zinc oxide (ZnO) is integrally fixed in the insulator with an inorganic adhesive agent.
There have been employed several kinds or types of lightning arresters in order to protect a power generating facility or plant, a substation and an insulator of the arrester itself against an excessive current or surge caused by a thunderbolt or lightning or for other reasons. A lightning arrester of the type disclosed in Japanese Patent Applications published under Laid-Open Nos. 124294/1979 and 32308/1980, wherein a voltage non-linear resistor having a major constituent of ZnO is integrally fixed in the insulator with an inorganic adhesive agent such as cement or glass, shows superior arresting characteristics, and has been in the limelight among other types of lightning arresters.
This known type of voltage non-linear resistor having a major constituent of ZnO has been improved in its resistance to deterioration by using a method wherein, as described in the above-identified prior publications, an intermediate layer of an inorganic adhesive agent such as cement or glass is interposed between the resistor and the inner surface of the insulator to reduce a surface area of the resistor contacting the surrounding air, in view of the fact that a resistance value of the resistor is gradually decreased under a reaction with a moisture, even when only a small amount is contained in the air and that a quantity of heat generated from the resistor is gradually increased, thereby producing a possibility of rupture of the insulator or other components of the insulator.
However, as described in the prior publications, the mere presence of such an intermediate adhesive layer, for example a glass layer between the insulator and the voltage non-linear resistor having ZnO as a major constituent, will not completely solve the prior problem; there are still the disadvantages that some cracks may be generated at interfaces between the adhesive layer and the insulator and/or the resistor of ZnO, with a result of possible destruction of the insulator leading to a serious accident. Such destruction, for example, may be due to a thermal stress which can be produced when the resistor of ZnO is rapidly cooled after its heat treatment during manufacture or by a rain or snow failing upon the insulator which has been heated by a charging of voltage or when the resistor is rapidly heated by lightning. Such thermal stress is caused by differences in physical properties such as the coefficient of thermal expansion, thermal conductivity and mechanical strength between the materials used.